Compare expanded preprompt when rendering (#214)

* Compare expanded preprompt when rendering

The expanded content of the preprompt was not considered previously when
checking if the content had changed. E.g. path changes were never detected.
The typeset (definition) and assignment are performed on separate lines as it
would cause issues otherwise.

Fixes #213.

pure.zsh

@@ -141,12 +141,15 @@ prompt_pure_preprompt_render() {
 	# execution time
 	preprompt+="%F{yellow}${prompt_pure_cmd_exec_time}%f"
 
+	# make sure prompt_pure_last_preprompt is a global array
+	typeset -g -a prompt_pure_last_preprompt
+
 	# if executing through precmd, do not perform fancy terminal editing
 	if [[ "$1" == "precmd" ]]; then
 		print -P "\n${preprompt}"
 	else
-		# only redraw if preprompt has changed
-		[[ "${prompt_pure_last_preprompt}" != "${preprompt}" ]] || return
+		# only redraw if the expanded preprompt has changed
+		[[ "${prompt_pure_last_preprompt[2]}" != "${(S%%)preprompt}" ]] || return
 
 		# calculate length of preprompt and store it locally in preprompt_length
 		integer preprompt_length lines
@@ -157,7 +160,7 @@ prompt_pure_preprompt_render() {
 
 		# calculate previous preprompt lines to figure out how the new preprompt should behave
 		integer last_preprompt_length last_lines
-		prompt_pure_string_length_to_var "${prompt_pure_last_preprompt}" "last_preprompt_length"
+		prompt_pure_string_length_to_var "${prompt_pure_last_preprompt[1]}" "last_preprompt_length"
 		(( last_lines = ( last_preprompt_length - 1 ) / COLUMNS + 1 ))
 
 		# clr_prev_preprompt erases visual artifacts from previous preprompt
@@ -190,8 +193,8 @@ prompt_pure_preprompt_render() {
 		zle && zle .reset-prompt
 	fi
 
-	# store previous preprompt for comparison
-	prompt_pure_last_preprompt=$preprompt
+	# store both unexpanded and expanded preprompt for comparison
+	prompt_pure_last_preprompt=("$preprompt" "${(S%%)preprompt}")
 }
 
 prompt_pure_precmd() {